Conventionally, a wire saw has been known as a means for slicing a workpiece such as a semiconductor ingot into wafers. The wire saw has a wire row formed by winding a wire for slicing around a plurality of grooved rollers multiple times, and slices a workpiece simultaneously at each wire position by causing the wire for slicing to travel in the direction of a wire axis at a high speed and feeding the workpiece with respect to the wire row while supplying a slurry properly.
Incidentally, the wire saw uses a wire composed of a material with high abrasion resistance, high tension resistance, and high hardness such as a piano wire and a grooved roller composed of a resin with a prescribed hardness, which prevents damage of the wire. Unfortunately, wear or fatigue of the wire over time may cause breaking of the wire during slicing of a workpiece, making it impossible to continue slicing of the workpiece.
In such a case, the following operations are usually preformed: disengaging operation that disengages the wire from a cut portion of the workpiece; unreeling operation that unreels the wire manually or unreels breaking portions of the wire to the outside of one grooved roller appropriately by manually operating a grooved roller drive to connect the breaking portions and then unreels the connected portion of the wire again to a position in which the connection is not directly involved in the slicing of the workpiece; replacing operation that replaces the wire with a new one, if the wire is unusable.
After such wire maintenance, return operation is performed such that each wire in the wire row is engaged with a corresponding cut portion of the workpiece, and the slicing of the workpiece is resumed to complete the slicing of the workpiece, whereby restoration operation is finished.
However, except when the time required to resume slicing of the workpiece after the start of restoration of the wire is extremely short, for example, when wire maintenance can be completed only by connecting the broken wire: the breaking of the wire occurs at a position where the wire does not fit into the grooved roller, wire maintenance for a long time (e.g., an hour or more) causes cooling and contraction of the grooved rollers, which have been thermally-expanded due to frictional heat generated between the grooved roller and its bearing or between the grooved roller and the wire; consequently the pitch of the wire row becomes narrower than that during the slicing. Resuming of the slicing of the workpiece in this state arises a problem in that an uncorrectable step is undesirably generated on a cutting plane of the sliced wafer.
To address this problem, Patent Document 1 discloses a method for operating a wire saw and a wire saw that resume slicing after adjusting axial displacement of grooved rollers and the temperature of a workpiece so that the axial displacement of the grooved rollers and the temperature of the workpiece become the same as those recorded at the time of suspension of slicing of the workpiece by supplying temperature adjusting media, each having separately adjusted temperature, to the grooved rollers and the workpiece.
These method and apparatus make it possible to avoid a step, which may be caused by contraction of the grooved rollers and the workpiece, from being generated on the wafer surface, even when it takes long hours to perform wire maintenance.